One use of actuator control systems wherein accurate, positive and reliable control is required, is in the regulation of liquid and/or gas flow. In this environment, actuators are utilized to position a valve or choke to regulate the flow of liquid through the valve. The controller is used to selectively position the valve between the open or closed position to regulate flow rate, differential pressure across the valve, upstream pressure, downstream pressure or one or more of the above factors. Control actuated valves are used in the petroleum well environment wherein operating parameters vary drastically and reliable and accurate control is critical. An example is in the use of choke valves in the production of petroleum products from wells. To maximize the efficiency in some wells, it is necessary to control the production of the well so that, for example, the well pressure does not fall below a minimum which would result in damage to the well's ability to produce. In others the flow rate is controlled to minimize sand contamination. A difficulty in continuously, accurately and reliably controlling the pressure or flow of the well can be seen when one considers the severe environmental factors which are present in a well. For example, the downhole or supply pressure of a producing well can vary severely and, therefore, any actuator system used to control the choke valve must be able to function reliably and accurately to position the valve.
In other uses, an actuator control choke valve is used during drilling to maintain a given working pressure while continuously circulating the drilling fluids. In these uses loss of circulation of drilling fluid can damage the well. These chokes operate at pressures as high as 20,000 pounds per square inch.
One conventional method of actuating a control valve is to utilize a motor such as a DC stepping motor. The forces necessary to actuate the valve through the entire operating pressure range require the use of a gear train or rachet to connect the motor to the valve stem. This motor can provide a positive incremental input into the gear train or rachet which input can be relatively independent of the operating pressures acting upon the stem of the choke valve. However, due to the inherent backlash in gear trains and rachets, error is induced into the accuracy of positioning the valve.
In an attempt to overcome the errors induced into a control valve by variations in well pressure, partially balanced stem valves have been designed. These semi-balanced stem valves have complicated valve, valve seat and stem designs and attempt to balance the pressures acting on the stem whereby variations in the forces acting upon the valve stem due to changes in operating pressure are minimized. In some prior art systems a hydraulic or pneumatic actuator is coupled to a semi-balanced stem valve and a pressure differential across the piston of the actuator is created to control the position of the actuator and valve. Since the actuator is directly coupled to the stem of the valve, the inherent error of the gear train is eliminated. However, since the actuator position is a function of the differential pressure across the piston of the actuator, forces induced on the piston of the actuator from the stem of the control valve by reason of variations in the flowing or controlled pressure induce positioning errors in the system.
In other systems, single acting spring bias hydraulic or pneumatic actuators are directly coupled to somewhat balanced stem valves and the pressure on one side of the actuator is regulated. Like the double acting systems, these systems are pressure controlled and variations in the control of the supply pressure as well as the dynamic forces acting on the valve trim and stem induce errors into the systems performance.